Reorganize mount/unmount code so it is easier to add Darwin-specific implementation

After these changes, in order to add Darwin bind-mount implementation, one only needs:
* Adjust HasBindMounts definition in mount.go
* Provide implementation in mount_darwin.go

There was no consensus on adding dependency on bindfs, that seems to be the only working solution for bind-mounts on Darwin as of today, in https://github.com/containerd/containerd/pull/8789, that's why the actual implementation is not added in current PR.

As a bonus, Linux FUSE-related code was moved to a separate file and possibly could be reused on FreeBSD, though this needs testing.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>

core/mount/mount_unix.go

@@ -1,4 +1,4 @@
-//go:build !windows && !darwin && !openbsd
+//go:build !windows && !openbsd
 
 /*
    Copyright The containerd Authors.
@@ -19,10 +19,13 @@
 package mount
 
 import (
+	"fmt"
 	"os"
 	"sort"
+	"time"
 
 	"github.com/moby/sys/mountinfo"
+	"golang.org/x/sys/unix"
 )
 
 // UnmountRecursive unmounts the target and all mounts underneath, starting
@@ -69,3 +72,64 @@ func UnmountRecursive(target string, flags int) error {
 	}
 	return nil
 }
+
+func unmount(target string, flags int) error {
+	if isFUSE(target) {
+		// TODO: Why error is ignored?
+		// Shouldn't this just be unconditional "return unmountFUSE(target)"?
+		if err := unmountFUSE(target); err == nil {
+			return nil
+		}
+	}
+	for i := 0; i < 50; i++ {
+		if err := unix.Unmount(target, flags); err != nil {
+			switch err {
+			case unix.EBUSY:
+				time.Sleep(50 * time.Millisecond)
+				continue
+			default:
+				return err
+			}
+		}
+		return nil
+	}
+	return fmt.Errorf("failed to unmount target %s: %w", target, unix.EBUSY)
+}
+
+// Unmount the provided mount path with the flags
+func Unmount(target string, flags int) error {
+	if err := unmount(target, flags); err != nil && err != unix.EINVAL {
+		return err
+	}
+	return nil
+}
+
+// UnmountAll repeatedly unmounts the given mount point until there
+// are no mounts remaining (EINVAL is returned by mount), which is
+// useful for undoing a stack of mounts on the same mount point.
+// UnmountAll all is noop when the first argument is an empty string.
+// This is done when the containerd client did not specify any rootfs
+// mounts (e.g. because the rootfs is managed outside containerd)
+// UnmountAll is noop when the mount path does not exist.
+func UnmountAll(mount string, flags int) error {
+	if mount == "" {
+		return nil
+	}
+	if _, err := os.Stat(mount); os.IsNotExist(err) {
+		return nil
+	}
+
+	for {
+		if err := unmount(mount, flags); err != nil {
+			// EINVAL is returned if the target is not a
+			// mount point, indicating that we are
+			// done. It can also indicate a few other
+			// things (such as invalid flags) which we
+			// unfortunately end up squelching here too.
+			if err == unix.EINVAL {
+				return nil
+			}
+			return err
+		}
+	}
+}